In recent years, super-clarification techniques for air, gases, water, chemicals, etc. based on the membrane separation techniques have made a remarkable progress and are widely used in various fields such as the fields of foods, semi-conductors and the like. Particularly, a chemicals clarification technique required for fine processing in the process for producing semi-conductor integrated circuits is behind in technical response as compared with the clarification technique for air, gases, water, etc., and it is recently attracting a special attention. As one of the main reasons for that, while increasing the purity of chemicals is being required in attaining an improvement in the yield and quality of products, both semi-conductor integrated circuit manufacturers and chemicals suppliers are behind in their technical response, there is given delay in the development of filter products which are less in metal leaching and good in chemicals resistance. As filters which meet the above requirements at present, ones made wholly of fluorocarbon resins are being required, and membrane filters of a multi-flat membrane system and a tubular form have been developed. But, these filters have not yet come to satisfy the needs of the market, because both effective filter area and flow rate of filtration are so small that there are caused problems in filtration of highly viscous chemicals and mass treatment.
On the other hand, a pleat-form filter element with fluorocarbon resin membranes has been developed. This filter element has a large effective filter area as well as a fairly superior chemicals resistance, but because of the adhesion of fluorocarbon resin being very difficult in itself, parts other than the filter membrane are made of materials other than fluorocarbon resin which are not sufficient in terms of chemicals resistance, temperature resistance and metal leaching.
Consequently, satisfactory products are not yet obtained.
Further, a filter employed in the foregoing process for producing semi-conductor integrated circuits requires pores as very fine as 0.01 to 10 .mu.m. Also, the filter membrane itself is very low in strength, so that severe conditions cannot be employed on filter production, and besides the members of the filter element must be assembled not by mere adhesion, but by strong adhesion in a completely sealed state.
U.S. Pat. No. 3,457,339 discloses a production of a tubular shape filter having a leakproof property by applying a thermoplastic end caps which inside face is fused to a filter sheet material formed in a tubular shape. However, in case that the end cap is a fluorocarbon resin, even if the inside face of the cap is fused at temperature above the melting point, the fused resin can not penetrate pleats of filter because of its high viscosity.
Consequently, U.S. Pat. No. 3,457,339 can not be applied to a filter element made wholly of fluorocarbon.